Documenting the Discoveries

The Treaty of Guadalupe Hidalgo that ended the war with Mexico in 1848 added more than 525,000 square miles of western territory to the United States—an area that now encompasses all or parts of eight states. Members of Congress, recognizing the potential of this territory for settlement, fiercely debated whether or not to allow slavery there and which route a transcontinental railroad should follow through it. To inform future legislation, Congress funded a full report on the region as part of the boundary survey.

After considerable debate, the U.S. Senate approved for ratification the Treaty of Guadalupe Hidalgo on March 10, 1848, by a vote of 38–14. This annotated Senate copy indicates controversial points in Article V, which designated an 1847 map by J. Disturnell as the basis for the U.S.-Mexico boundary. The Senate rejected a clause limiting changes to that boundary.

Records of the U.S. Senate, National Archives and Records Administration

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Sketch from the Office of the Mexican Boundary Survey, n.d.

This area south of the Rio Gila, sketched by U.S. Boundary Commissioner William H. Emory, is now part of Arizona. The red line shows the U.S.-Mexico border before the United States purchased the adjacent land from Mexico in 1853. Secretary of War Jefferson Davis gave Army engineers this map to explore a southern railroad route to the Pacific.

Records of the U.S. House of Representatives, National Archives and Records Administration

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Report on the U.S. and Mexican Boundary Survey, 1857-59, directed by William H. Emory, Vol. 1, Vol. 2: Parts 1 and 2, 1859

Members of Congress relied on the U.S. Boundary Commission report as they proposed legislation for the new territory. Richly illustrated, the multivolume report included contributions by scientists along with maps of the new U.S.-Mexico boundary. These pages describe the Pecos River, depict an Apache warrior, and record the coyote skull seen in the adjacent case.